1. Field of the Invention
The present invention relates to a low-friction sliding member, which is used under a wet condition employing a lubricant, which exhibits low friction coefficient between itself and mating members, and which is good in terms of the wear resistance.
2. Description of the Related Art
It is required for sliding members, such as pistons and valve-system component parts making engines, to reduce energy loss resulting from friction as much as possible in view of resource conservation and environmental protection. Accordingly, the sliding surfaces of sliding members have been subjected to various surface treatments conventionally in order to reduce the friction coefficient between sliding members and mating members, and to improve wear resistance. In particular, amorphous hard carbon films called diamond-like carbon (hereinafter abbreviated to as “DLC”) films have been utilized widely as films which enhance the slidablity of sliding surfaces.
For example, Japanese Unexamined Patent Publication (KOKAI) No. 3-240,957 discloses an amorphous hard carbon film which includes silicon (Si). This conventional amorphous hard carbon film exhibits low friction coefficient between itself and mating members under a dry condition employing no lubricant. However, it is difficult for the conventional amorphous hard carbon film to reduce friction coefficient under a wet condition employing a lubricant. One of the causes of the disadvantage is believed to be the influence of a variety of additives included in lubricants. Additives included in lubricants adsorb onto amorphous hard carbon films' surface and react therewith to form boundary films. Hence, it is believed that boundary films, which are formed in sliding operations, determine actual friction coefficients. In particular, when oils for driving systems, oils which are intended for coping with high friction for power transmission, are used as lubricants, it is difficult to reduce actual friction coefficient in sliding operations.
On the other hand, Japanese Unexamined Patent Publication (KOKAI) No. 2001-192,864 discloses an attempt to use a lubricant which contains an aromatic compound. Aromatic compounds form firm boundary films on the surfaces of amorphous hard carbon films, because they exhibit high adsorption forces to amorphous hard carbon films. That is, in this conventional attempt, a firm boundary film is formed on the surfaces of amorphous hard carbon films in order to diminish the proportion of solid contact and eventually reduce actual friction coefficients in sliding operations. However, when an aromatic compound, an additive, is changed to another additives other than aromatic compounds, the another additives adsorb onto and react with amorphous hard carbon films so that the conventional attempt might suffer from hindered reduction of actual friction coefficients in sliding operations. Moreover, in view of environmental protection, it is believed as well that reviewing the types of usable additives and optimizing their usable amounts would get underway greatly. If such is the case, it is expected that it would be difficult for the conventional attempt, which relies on additives' adsorption and reaction to amorphous hard carbon films, to reduce actual friction coefficients in sliding operations.